The present invention relates to a display apparatus for displaying a plurality of electrical signals and more particularly to a raster-scanned terminal display device such as a cathode-ray tube in which a single line may be selected and positioned with respect to a plurality of fixed-position lines.
One form of information display apparatus, the oscilloscope, has the capability of displaying several electrical signals at substantially the same time, wherein a plurality of amplifier channels are enabled on a time-sharing basis known in the art as chopping and alternate display modes. For this type of operation, an amplifier channel is required for each signal, each amplifier channel having a position control to establish the vertical position of the baseline, or horizontal trace associated therewith. It can be readily appreciated that this system, while permitting display of as many as eight channels of information, has many drawbacks. Each amplifier channel having independent controls must be connected to a signal source, resulting in a multiplicity of cables, probes, or signal leads. Each signal, its amplitude, and its relative display vertical position must be kept track of, leading to confusion and error. The confusion becomes particularly noticeable if all the signals are similar, such as digital logic waveforms as opposed to analog waveforms.
Waveform recorders and logic analyzers are presently available which are capable of storing the digital equivalents of a plurality of electrical waveforms. The recorded waveforms may then be accessed individually on a time-sharing basis and applied via a single amplifier channel to a utilization device, such as an oscilloscope, X-Y plotter, strip chart recorder, computer, or even another storage device. When particularly applied to an oscilloscope or a cathode-ray tube display monitor, the waveforms may be applied one line at a time in a fixed raster format at a rate fast enough to produce a multiple-line flicker-free display. This is achieved by applying a stairstep signal to the vertical deflection plates, each step corresponding to a line and having the waveform superimposed thereon for display. In synchronism with each stair step, a sawtooth signal is applied to the horizontal deflection plates to provide a time base sweep. The resultant multiple-line display of several waveforms fixedly spaced from one another is produced at a rate higher than the critical fusion frequency of the human eye, permitting flicker-free simultaneous viewing thereof.
While several waveforms may be examined simultaneously in this manner, it is difficult to ascertain timing relationships between non-adjacent lines. One attempted solution to this problem was the superposition of precisely spaced triggers on the waveform, coupled with a capability of expanding the time scale to permit examination of a relatively short time duration. However, a major drawback of this mode of operation is that as the time scale is expanded, the fraction of viewable waveforms becomes smaller, reducing to nothing more than a series of straight lines at which the operator must guess are high or low, and leading and trailing edges which deprive the operator of a comprehensible perspective of the displayed information.